Accurate control over the torque applied to threaded fasteners is of increasing importance in assembly operation. Numerous devices have been utilized in the past to shut off the power supply or to disengage the power tool from the fastener at a predetermined torque output. The resulting tension produced in the fastener generally has not been within the required tolerances, or at a sufficiently high level for maximum fastener utilization. Several methods have been developed recently to improve the level of fastener utilization by taking the fastener to a relatively high tension level, including to its yield point. The methods disclosed to date have required relatively sophisticated electronic apparatus to accomplish their function. Most of these methods have required the use of output torque transducers and angular position encoders coupled with differentiating circuitry either mechanical or electrical.
The present invention obviates the need for elaborate equipment. It is well known in the art that after the threaded fastener has been run down and firmly seated, the tension in the fastener becomes proportional to the further rotation of the fastener until the fastener or its mating threads begin to yield. At this point, further rotation of the fastener will not result in a proportional increase in the tension in the fastener. In addition, as the fastener is rotated, the torque applied to the fastener increases proportionately until the yield point is reached. At the yield point, an increase in the applied torque will result in greater fastener rotation than when the fastener is in its proportional range. It is this occurrence on which the present invention is predicated.